An organic EL device having an organic light emitting layer interposed between electrodes has been conventionally researched and developed in an intensive manner owing to, for example, the following reasons:    (1) the organic EL device can be easily handled and produced because it is a complete solid-state device;    (2) the organic EL device does not require any light emitting member because it can spontaneously emit light;    (3) the organic EL device is suitable for a display because it is excellent in visibility; and    (4) the organic EL device facilitates full colorization.
In general, a fluorescent emission phenomenon (luminescence phenomenon) as energy conversion occurring when a fluorescent molecule in a singlet excited state (which may hereinafter be referred to as the “S1 state”) in an organic light emitting medium undergoes radiative transition to a ground state is used as the mechanism via which such organic EL device emits light. In addition, a fluorescent molecule in a triplet excited state (which may hereinafter be referred to as the “T1 state”) is also assumed in the organic light emitting medium. However, such fluorescent molecule gradually undergoes non-radiative transition from the triplet excited state to any other state because its radiative transition to a ground state is forbidden transition. As a result, heat energy is released instead of the occurrence of fluorescent emission.
The terms “singlet” and “triplet” as used herein each refer to the multiplicity of energy determined by the number of combinations of the total spin angular momentum and total orbital angular momentum of a fluorescent molecule. That is, a singlet excited state is defined as an energy state in the case where a single electron is caused to undergo transition from a ground state with no unpaired electron to a higher energy level while the spin state of an electron remains unchanged. In addition, a triplet excited state is defined as an energy state in the case where a single electron is caused to undergo transition to a higher energy level while the spin state of an electron is reversed. Of course, light emission from the triplet excited state thus defined can be observed at an extremely low temperature such as the temperature at which liquid nitrogen liquefies (−196° C.). However, the temperature condition is not practical, and the quantity of emitted light is slight.
By the way, the total luminous efficiency of a conventional organic EL device is related to the efficiency (Φrec) with which injected charge carriers (an electron and a hole) recombine with each other and to the probability (Φrad) that a produced exciton causes radiative transition. Therefore, the total luminous efficiency (Φe1) of an organic EL device is represented by the following equation.Φe1=Φrec×0.25Φrad 
Here, “0.25” of the coefficient for Φrad in the equation is determined on the basis of the assumption that the probability for producing a singlet exciton is ¼. Therefore, a theoretical upper limit for the luminous efficiency of an organic EL device is 25% even when it is assumed that recombination and the radiation damping of an exciton each occur at a probability factor of 1. As described above, in the conventional organic EL device, no triplet exciton can be substantially utilized, and only a singlet exciton causes radiative transition, so there arises a problem in that an upper limit value for luminous efficiency is low. In view of the foregoing, attempts have been made to cause a fluorescent emission phenomenon to occur even under a room temperature condition through the transfer of energy from a produced triplet exciton to a phosphorescent dopant by utilizing a triplet exciton (triplet excited state) of an organic light emitting material (host material) (see, for example, Non-patent Document 1). To be more specific, it has been reported that a fluorescent emission phenomenon is caused by constituting an organic EL device including an organic light emitting layer constituted by 4,4-N,N-dicarbazolylbiphenyl and an Ir complex as a phosphorescent dopant.
Under such circumstances, research on a phosphorescent device utilizing a triplet exciton has been recently advanced. For example, Patent Documents 1 to 3 each disclose, as a host compound, a compound using a naphthyl group or an anthranyl group as a linking group for a carbazolyl group. In addition, Patent Document 4 discloses, as host compounds for a blue color, a specific group of compounds in each of which a triazine ring and a carbazolyl group couple with each other. However, the performance of each of organic EL devices using those compounds is not sufficient for practical use, and no compound containing a condensed ring such as a naphthyl group or an anthranyl group in any one of its molecules has been disclosed as a host material for red-based color phosphorescence.
Patent Document 1: JP-A-2001-072927
Patent Document 2: JP-A-2003-077674
Patent Document 3: JP-A-2003-031371
Patent Document 4: JP-A-2002-193952
Non Patent Document 1: Jpn. J. Appl. Phys., 38(1999)L1502